While silver halide photographic elements are capable of directly recording X ray exposures, they are more responsive to light within the visible spectrum. It has become an established practice to construct Duplitized .RTM. (double coated) radiographic elements in which silver halide emulsion layers are coated on opposite sides of a film support and to sandwich the radiographic element between intensifying screen pairs during imaging. The intensifying screens contain phosphors that absorb X radiation and emit light. This light is transmitted to the silver halide emulsion layer on the adjacent face of the film support. The result is that diagnostic radiographic imaging is achieved at significantly reduced X ray exposure levels.
An art recognized difficulty with employing double coated radiographic elements as described above is that some light emitted by each screen passes through the transparent film support to expose the silver halide emulsion layer on the opposite side of the support to light. This results in reduced image sharpness, and the effect is referred to in the art as crossover.
A variety of approaches have been suggested to reduce crossover, as illustrated by Research Disclosure, Vol. 184 August 1979, Item 18431, Section V. Cross-Over Exposure Control. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire p010 7DD, England.
One approach to reducing crossover has been to dissolve a filter dye in one or more of the hydrophilic colloid layers forming the radiographic element. Such dyes must, of course, be selected to minimize residual density (stain) in the image bearing radiographic element. A pervasive problem with dissolved dyes has been their migration to the latent image forming silver halide grains, whether coated directly in the image forming emulsion layers or in underlying layers. This has resulted in loss of photographic speed, which, of course, runs directly counter to the general aim in adopting a double coated radiographic element format in the first instance. Thus, where this approach has been followed, a balance of reduced photographic speed and residual crossover has been accepted. Although mordants have been employed to reduce dye migration, they have not been effective in preventing loss of photographic speed and have further proved disadvantageous in increasing the bulk of the water permeable layers of the radiographic elements, thereby increasing the processing time required to produce a processed element that is dry to the touch. The dissolved dye approach to crossover reduction is illustrated by Doorselaer U.K. Pat. Spec. 1,414, 456 and Bollen et al U.K. Pat. Specs. 1,477,638 and 1,477,639.
To reduce dye migration to the image forming silver halide grains a variant approach has been to adsorb the dye to the surfaces of silver halide grains other than those employed in imaging. This approach reduces speed loss, but has the disadvantage of requiring silver halide grains to be present in addition to those required for latent image formation. Further, an added silver halide grain population increases vehicle requirements and correspondingly increases drying times. Millikan et al U.K. Pat. Spec. 1,426,277 illustrates this approach applied to a specialized photographic imaging system in which a silver halide grain population is present in addition to the grain population which is relied upon to produce a latent image.
The most successful approach to crossover reduction yet realized by the art has been to employ double coated radiographic elements containing spectrally sensitized high aspect ratio tabular grain emulsions or thin intermediate aspect ratio tabular grain emulsions, illustrated by Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426, respectively. Crossover levels below 20 percent (but well above 10 percent) are reported.